As described in U.S. Pat. application Ser. No. 07/584,025, the subject matter of which is incorporated herein by reference, in recent years, methods and apparatus for creating evoked potentials in the neural pathways of higher level organisms (e.g., animals and humans) have been developed. Evoked potentials can produce observable movements and/or analyzable electrical signals (e.g., brain waves). Evoked potentials are created by stimulating neural pathways. One type of stimulator is an electrical stimulator. There are two types of electrical stimulators. One type is formed by a pair of spaced-apart electrodes. When electric potential is applied to the electrodes a current flow through the body is created. The current flow produces an electrical field that disrupts the polarization of neurons located in the field (commonly called depolarization of the neurons) causing an evoked potential "message" to be transmitted along the neural pathways formed by the depolarized and other neurons that define the neural pathway.
While electrical stimulation using a pair of spaced-apart electrodes has certain advantages, it has several disadvantages. One main disadvantage of electrode stimulation relates to the fact that the body is an insulator. As a result, the electrical current flow between the electrodes is shallow, i.e., it occurs near the skin. Because current flow is shallow, the electrical field created by the current flow is shallow. Thus, deep neurons are not depolarized and, thus not stimulated. While current can be increased to increase current penetration depth and, thus, stimulation depth, higher current flows cause pain and, thus, are undesirable. In fact, pain is one of the major reasons why the use of electrode stimulators to stimulate the brain can only be used on comatose patients. Awake patients generally cannot stand the pain associated with the high current flow needed to stimulate neurons enclosed by cranial bone.
In order to overcome the shallow penetration disadvantage of electrode stimulators, magnetic stimulators have been developed. Magnetic stimulators use a magnetic coil to create a neuron depolarizing magnetic field. Magnetic stimulators h ave the advantage of being relatively pain free and noncontacting, as well as being capable of stimulating deep and otherwise inaccessible neurons Depth is improved because, unlike current flow, body tissue does not resist magnetic flux. While magnetic stimulators have the ability to provide deeper penetration with less pain, as is best understood, neuron depolarization is still due to the creation of an electric field. More specifically, as best understood, the changing magnetic field created by a magnetic stimulator induces eddy currents in body tissue that, in turn, create a neuron depolarizing magnetic field.
While prior magnetic stimulators have been used to stimulate neurons in a relatively painless manner deeper than the neurons can be stimulated by electrode stimulators, the depth of penetration is still less than desired, particularly in the cranial area. The present invention is directed to providing a magnetic stimulator that is capable of stimulating deeper and otherwise inaccessible neurons in the cranial area.